High efficiency bridge amplifier

Abstract: Distorting effects and disturbances caused by abrupt voltage changes on the output nodes of a pair of selfconfiguring amplifiers for alternatively driving in a bridge mode or in a single-ended mode by one amplifier of the pair a certain load, in function of the level of the input signal, because of the enabling/disabling of a common mode control loop, are effectively eliminated by storing the voltage of a common mode control node of the pair of amplifiers on a capacitance during a phase of disabling of said common mode control loop. Abrupt voltage drops and recoveries at the switching instants are prevented.

Description: The present invention relates to a selfconfiguring amplifier wherein voltage spikes on the outputs arising from the switching from a single-ended to a bridge configuration and viceversa of a pair of amplifying stages are effectively depressed. In many applications and primarily in audio reproduction systems, for example in car radios and similar apparatuses that are intrinsically compact because of stringent installation requirements, as well as in portable apparatuses, power dissipation in final amplifying stages, often quadruple in order to drive a pair of loudspeakers (front and rear) for each stereo channel, may create problems of heat balance. For example, four 20W amplifiers may have a power dissipation of about 4x12=48W and because of the limited space available in certain apparatuses, such a power may be difficult to be dissipated without an increased internal temperature of the apparatus. On the other hand, a relatively high temperature of operation may damage the magnetic tape of cassettes or optical disks (CD), the drives of which are often crammedly fitted inside a single apparatus case. The absence of a double (or symmetrical) supply in many of these apparatuses does not allow the use of highly efficient configurations in terms of power dissipation, as for example a so-called class-G configuration. With the aim of overcoming these limitations or, notwithstanding these limits reducing somewhat power dissipation for the same audio power output, particular selfconfiguration techniques of several integrated amplifiers have been developed for optimizing power dissipation by modifying their configuration in function of the signal level. Systems of this type are described in the European patent application EP 0 487 133 A, filed on November 9, 1991, and in the European patent application EP 0 613 242 A, filed on February 24, 1993, both in the name of the same applicant. As mentioned at the end of the description of the selfconfigurable double bridge amplifier disclosed in the prior European patent application EP 0 613 242 A, during operation phases in which an amplifier of each pair of amplifiers is configured so as to function as a single-ended amplifier for driving its own load in series with the load of the other amplifier of the other pair, the common mode control loop of each pair of configurable bridge amplifiers is automatically disactivated, according to well known techniques. An example of a common mode control loop that can be activated during a phase of bridge configuration of the amplifiers is described and illustrated in prior European patent application EP 0 487 133 A. In Figures 4 and 5 of said prior patent application is shown a typical common mode control loop constituted by a voltage divider R6-R7 and a differential amplifier TR suitable to force the maintainment of a constant voltage on a common output node C of the pair of amplifiers A1 and A2. The common mode control loop is activated or disactivated by the configuring switch F. In practice the loop is disactivated when the second amplifier A2 is disactivated and the amplifier A1 drives the load L in a single-ended mode. Abrupt voltage changes occurring at every transition from a configuration to the other, cause distortions that are hardly recovered by the feedback system of the amplifier because they encompass a range of high frequency harmonics, where the gain of the feedback loop of the amplifier and therefore the reduction of distortion is relatively modest. Similar abrupt variations of the voltage would occur also in the case a disabling switch of the common mode control loop is used at the output of the common mode feedback amplifiers (Gm1, Gm2) instead of using a switch for shortcircuiting its inputs as in the scheme shown in Figures 4 and 5 of the prior European patent application EP 0 487 133 A. Another negative effect of these abrupt voltage changes is represented by the consequent electromagnetic disturbance (emi) they may generate. SUMMARY OF THE INVENTION These problems and limitations of the known systems are overcome by the present invention which is defined in the claims. It has been found that the disturbances caused by switchingly enabling and disabling a common mode control loop of a pair of selfconfiguring amplifiers may be substantially eliminated. This is achieved by implementing a different method of enabling/disabling the common mode control loop. According to the invention, enabling/disabling of the common mode control loop is effected by employing a configuring switch and a storage capacitance for retaining a common mode voltage information during the period in which the configuring switch remains open. Substantially this maintains the voltage of the common mode control node at the same value it had at the last switching instant. In this way, large and abrupt voltage transitions are prevented in the output signals of the configurable amplifiers. The elimination of the problems connected with the abrupt voltage variations provoked by the configuring switchings, that is by the alternating enabling/disabling of the common mode control loop, is achieved with a negligible increase of the complexity of the circuit of the system of selfconfiguring amplifiers as a whole, because the storage capacitances may be very small and therefore easily integratable. BRIEF DESCRIPTION OF THE DRAWINGS The different aspects and advantages of the invention will become more evident through the following description of several important embodiments and by referring to the annexed drawings, wherein: Figure 1 shows a functional circuit of a double bridge amplifier provided with respective common mode control loops without disturbance suppressing means; Figure 2 shows a functional circuit similar to the one depicted in Fig. 1, including means for suppressing disturbances due to the configuring switchings; Figures 3a and 3b show comparable waveform as produced in the circuit of Fig. 1 and in a circuit of Fig. 2; Figure 4 shows the coupling of the output of a common mode control loop more in detail; Figure 5 shows the circuit diagram of a common mode feedback amplifier usable in a selfconfiguring amplifier of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS The circuit diagrams of Figures 1 and 2 represent a selfconfiguring, double bridge amplifier, for example for a stereo channel of an audio reproduction installation. The gist of this invention will be better illustrated by first analyzing a circuit without disturbances suppressing means (Fig. 1, Fig. 3a) and thereafter with disturbances suppressing means (Fig. 2, Fig. 3b). The respective common mode control loops of each of the two pairs of amplifiers (OPA+F_OPA-F and OPA-R_OPA+R) are also shown in the circuit diagram of Fig. 1. Functionally, each common mode control loop comprises a resistive voltage divider RC1-RC2 (RC3-RC4), a common mode differential amplifier Gm1 (Gm2) and a configuring switch SW_CL1 (SW_CL2). It has been observed that such a common mode control system produces some important drawbacks. When the system functions with the configuring switches SW_F open and SW_C closed, that is when the double bridge amplifier is configured for a relatively low output power (low level of the input signal) by the configuring window comparator, the common mode feedback amplifiers Gm1 and Gm2 are practically disactivated by their respective configuring switches SW_CL1 and SW_CL2 which shortcircuit the input terminals of the respective feedback amplifier. When, upon the rise of the input signal level, the configuring window comparator commands the configuring switches to implement a double bridge structure, that is with SW_F closed and SW_C open, the relative switches SW_CL1 and SW_CL2 switch to ground potential. As a consequence, each common mode amplifier, for example Gm1, acts on the respective pair of operational amplifiers OPA+F and OPA-F which compose a single bridge driving a respective load (loudspeaker), so as to bring the voltage (Vc1) of the common node of the resistances RC1 and RC2 to ground potential. In the waveforms shown in Fig. 3a, it may be observed that these transitions of the inverting input of the common mode feedback amplifier cause abrupt voltage variations on the outputs of the amplifiers (instants labelled A, B, C and D in the diagrams) when the node Vc1 switches from a certain potential assumed before the switching to ground potential. According to this invention, these abrupt voltage changes may be avoided by modifying the manner in which the selfconfiguring system operates (enables and disables the relative common mode control loop) and precisely by storing the actual voltage of the common mode control node (Vc1), assumed during a phase in which the common mode control loop is disactivated, that is when the two inputs of the common mode feedback amplifier (Gm1) are shortcircuited, on a storage capacitor connected between the inverting input of the feedback amplifier at ground. In this way, the opening and closing of the input shortcircuiting switch will not generate any abrupt voltage change at these input nodes. The system of the invention may be fully implemented as depicted in Fig. 2 by introducing a storage capacitor CM1 (CM2) that may be connected (as shown in the example of Fig. 2) between the inverting input of the common mode feedback differential amplifier Gm1 (Gm2) and ground, and by employing a simple configuring switch SW_CL1 (SW_CL2) for shortcircuiting the inverting input with the noninverting input of the common mode feedback amplifier when the selfconfiguring system commands a bridge configuration of the pair of amplifiers OPA+F and OPA-F (OPA-R and OPA+R). Of course, the storing capacitor CM1 (CM2) and obviously also the relative configuring switch SW_CL1 (SW_CL2) may be connected alternatively to the other (noninverting) input of the common mode feedback differential amplifier Gm1 (Gm2). Also in this case, the capacitor stores the common output node Vc1 (Vc2) during a closing phase of the configuring switch. By referring to any pair of bridge configurable amplifiers, for example to the pair composed of OPA+F and OPA-F, during a phase in which the switch SW_CL1 is closed thus shortcircuiting the inputs of the common mode feedback amplifier Gm1, the storage capacitor Cm1 charges to the voltage of the common mode control node Vc1, which corresponds to the intermediate node of the resistive voltage divider composed of the resistances RC1 and RC2 of the common mode control network. The storage capacitor Cm1 retains this voltage information for the whole period during which the configuring switch SW_CL1 remains open and therefore acts on the common mode control loop so as to maintain the potential of the common mode control node Vc1 at the same value it had at the switching instant. As may be observed from the series of waveforms of Fig. 3b, the system of the invention eliminates any abrupt changes of the output voltages in correspondence of the switching instants A, B, C and D. The device of the invention can be easily realized because the value of the storage capacitance Cm1 may be very small and therefore can be easily integrated. Of course, the considerations made for the common mode control loop of one of the two selfconfiguring pairs of amplifiers, are valid also for a similar common mode control loop of the other pair of amplifiers that composes the selfconfiguring, double bridge amplifier shown in the figures. The coupling of the output of the common mode feedback differential amplifier to the respective configurable amplifier of the pair of amplifiers may be implemented in various ways. A particularly effective embodiment is shown in the partial circuit of Fig. 4. In practice, the resistances, R2 and R3 respectively, of the feedback loop of the operational amplifier OPA+F of one pair of amplifiers, which can be configured to function in a single ended mode of the functional diagram of Fig. 2, may be subdivided by employing pairs of resistances R2-R2' and R3-R3', as shown in Fig. 4. The common node of the resistances R2' and R3', which is connected to the inverting input of the OPA+F operational amplifier, is also connected to the output of the common mode feedback amplifier Gm1. In case of an integrated implementation of the amplifier, the ability of retaining the voltage information of the storage capacitor Cm1, which must necessarily require a small area of integration and therefore has a relatively small capacitance, may be enhanced by minimizing the input currents of the common mode feedback amplifier Gm1. To this end, field effect input transistors (MOSFET) are preferably used. A suitable common mode differential feedback amplifier circuit is schematically shown in Fig. 5.

Claims: A common mode control circuit for a pair of amplifiers (OPA+F_OPA-F, OPA+R_OPA-R) selfconfiguring in a bridge configuration for driving a first load and in a single-ended mode of operation of one of said amplifiers (OPA+F; OPA-R) for driving said first load, in function of the level of an input signal (IN-F; IN-R), comprising at least a common mode feedback differential amplifier (Gm1, Gm2) having a noninverting input (+) connected to a common mode control node (Vc1, Vc2) of said pair (OPA+F_OPA-F, OPA+R_OPA-R) of amplifiers and an inverting input (-) that is switched by configuring means, characterized by comprising a configuring switch (SW_CL1, SW_CL2) shortcircuiting said inverting input to said noninverting input; a storage capacitor (CM1, CM2) connected between any one of said two inputs (+, -) and a common potential node of the circuit of the amplifiers. The common mode control circuit as defined in claim 1, wherein said common mode feedback differential amplifier (Gm1, Gm2) comprises a pair of field effect input transistors. The common mode control circuit as defined in claim 1, wherein said storage capacitor (CM1, CM2) is connected to the inverting input (-) of said common mode amplifier. The common mode control circuit as defined in claim 1, wherein said one amplifier (OPA+F) of the pair (OPA+F_OPA-F) is configured in a single-ended operation mode to drive in a bridge mode said first load in series with a second load of a similar pair of selfconfiguring amplifiers (OPA+R_OPA-R), in function of the level of at least one of the input signals (IN_F, IN_R). A method for preventing abrupt voltage changes at the outputs of a pair of amplifiers (OPA+F_OPA-F, OPA+R_OPA-R) selfconfiguring in a bridge configuration for driving a first load and in a single-ended mode of operation of one of said amplifiers (OPA+F, OPA-R) for driving said first load, in function of the level of an input signal (IN_F, IN_R) said pair of amplifiers, providing at least a common mode feedback differential amplifier (Gm1, Gm2) having a noninverting input (+) functionally connected to a common mode control node (Vc1, Vc2) of said pair of amplifiers and an inverting input (-) that is switched by configuring means, characterized by storing the output common mode voltage of said pair of amplifiers on a capacitor (CM1, CM2) connected between an input (+, -) of said common mode feedback differential amplifier (Gm1, Gm2) and a common potential node of the circuit of the amplifiers, during a phase of bridge configuration when said inputs (+, -) are shortcircuited by a configuring switch (SW_CL1, SW_CL2) ; exploiting the voltage information stored in said capacitor (CM1, CM2) for keeping constant the voltage on said common mode control node (Vc1, Vc2), during a phase of single-ended configuration when said shortcircuiting switch (SW_CL1, SW_CL2) is open.

Inventors: BOTTI EDOARDO, MANDRINI TIZIANA, BOTTI, EDOARDO, MANDRINI, TIZIANA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H03F3
IPC codes: H03F3, H03F1